Author 



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Title 



Class....LJ 

Book --.-^.Za-jcLX- 



Imprint 



16— 47372-1 OPO 



DEVELOPMENT OF AN AIRCRAFT 
INCIDENCE METER 



M ( V BY 
A. F.' ZAHM, Ph.D. 

Navy Department, Bureau of Construction and Repair 



Reprinted from the Journal of The Franklin Institute 
November, 1919 




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PRESS OF 

J. B. LIPPINCOTT COMPANY 
1919 




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(Reprinted from the Journal of The Franklin Institute 

November, 1919 ) 

DEVELOPMENT OF AN AIRCRAFT INCIDENCE 

METER.* 



BY 



A. F. ZAHM, Ph.D. 

Navy Department, Bureau of Construction, and Repair. 

Preface. — To enable the air pilot to read at a glance the direc- 
tion of flow of the air past his airship or airplane, a balanced 
weathervane indicating promptly small changes of incidence has 
been developed and tried under regular working conditions. The 
scale drawings and test of the device herein described were made 
respectively by Mr. L. H. Crook and Mr. S. S. Rathbun, members 
of the aeronautics staff at the Washington Navy Yard. 



Fig. i. 




Aircraft incidence meter. 

Model — Figs. 1 and 2 give the general appearance and di- 
mensions of this instrument. It consists of a two-blade weather- 
vane supported on a horizontal pivot at the end of a bracket arm 
protruding forward from an airplane strut and adjustable in pitch 
by means of the clamping nut at its base. The vane has a forward 
counter-weight to insure static balance, and a pointer playing on a 
graduated arc of fourteen inches radius, indicating even degrees 



* Communicated by the Author. 



683 



68 4 



A. F. Zahm. 



[J.F.I. 



and readable to fractions of a degree from the pilot's seat. The 
blades have the sectional shape of an Eiffel Wing- No. 5, which 
at zero incidence possesses very slight drag and a large increase of 
lift with slight increase of incidence. 



Fig. 2. 




Plan for aircraft incidence meter. 

Wind Tunnel Test. — When the instrument was given its pre- 
liminary test in the 8' x 8' tunnel its pointer remained steadily 
fixed in the wind direction until forcibly displaced. It then 
promptly returned to zero incidence without lag or indication 
of friction. 



Observations with C. & R. Incidence Meter in Flight. 



Speed of Flight in Knots 


55 


65 


75 


Normal variation 

Precision of reading 

possible 

Occasional variation . . 


± 0.6° 

± 0.3° 

± 2.° 


± O.4 
± 0.2° 
± I.° 


± O.4 
± 0.2° 
± I.° 



Nov., igiy.] An Aircraft Incidence Meter. 685 

Test in Flight. — The instrument was finally mounted midway 
between planes on the nearest right-hand strut of Flying Boat 
HS-2 No. 1840, and carried through very still air at three different 
fixed speeds. The preceding table indicates its behavior under 
these circumstances. 

Conclusion. — If this instrument is to be put into use, it may 
be lightened somewhat and provided with a strap to lash its flange 
to the aeroplane strut. So finished, it would weigh about 
1.5 pounds. 



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